1. Field of the Invention
This invention relates to viscous shear couplings and to plates therefor. Such couplings find particular, but not exclusive, use in the drive transmissions of motor vehicles.
2. Description of Prior Art
A viscous shear coupling, as described for example in United Kingdom Patent Specification 1,357,106 (and corresponding U.S. Pat. No. 3,760,922 issued 25th Sept. 1973) comprises a housing part, a hub part within the housing part, the parts being relatively rotatable about a common axis, and two sets of interleaved planar plates which are provided with slots or holes. The plates of one of the sets are rotatable with the housing part and the plates of the other set are rotatable with the hub part. The plates of one of the sets are axially movable into contact with the plates of the other set. The housing part contains a viscous liquid such, for example, as a silicone oil.
Normally torque is transmitted between the housing part and the hub part due to shearing forces in the viscous liquid between adjacent plates, i.e. in viscous mode. The above patent specifications describe that the housing part may be filled with viscous liquid to a fill factor which leaves part of the housing normally filled with air or gas.
Such couplings exhibit a phenomenon which has come to be known as the "hump" mode in which the plates of the two sets come into contact and torque is transmitted by metallic friction and/or mixed friction between the plates.
Prolonged slipping of the coupling in viscous mode causes heating and expansion of the silicone oil. When the oil has expanded to fill the entire space within the coupling there is a sharp increase in pressure in the coupling and, due to the throttling effect between adjacent plates, the plates of one set are moved into contact with the plates of the other set by the fluid pressure in the coupling.
It has been proposed in DE-PS 3632283 to form the apertures in plates for viscous shear couplings in such a way that the apertures have thickened edges. These edges act as scraping edges to scrape off the viscous liquid when the coupling approaches the hump mode and therefore facilitate the transition of the coupling from viscous mode to hump mode.